Undesirable flow of a substance, such as a gas or liquid, into or out of a system is known as leakage. When the substance encounters friction, sound is generated. For small leaks, this sound is mostly in the ultrasonic range of frequencies, i.e. 20 kHz and higher, although a portion of its spectrum may also be in the audible range. Leaks occur through openings or "orifices" in the system which carries the gas or fluid. Normally these orifices are not smooth, well-defined holes, but rather cracks with many jagged edges and internal chambers. Fluid or gas escaping through such an orifice is forced into turbulence and this turbulent flow generates sound. The intensity of sound generated by a leak is a complex function of various factors such as viscosity, temperature, speed of the leak, the pressure differential across the leak and the physical characteristics of the orifice.
A variety of different signal detection instruments have been developed in the past in an effort to ascertain the presence of leaks. Many of these detection instruments include electronic circuits incorporating tuning circuitry adapted to receive, filter and process signals within a selected frequency range. For signal detectors specifically constructed to monitor a broad range of oscillatory signals, many employ super heterodyning in order to increase sensitivity, eliminate noise and shift frequency bands. One such signal detection instrument capable of ascertaining the presence of leaks in compressed air systems, or in most other pressurized fluid systems, is known as an ultrasonic leak detector. Ultrasonic leak detectors are specially adapted to detect ultrasonic signals created by the escape of pressurized gases through orifices. This is useful, for example, in detecting leakage from pipelines as well as detecting air flow paths through structural insulations and through automobile doors and panels. In these circumstances, sound generated at the leak point becomes airborne and travels to the detector's sensor, sometimes referred to as an airborne sensor, where it is converted into an electrical signal for further processing. Instruments particularly adapted for the detection of airborne sound are described in my U.S. Pat. No. 5,103,675 issued Apr. 14, 1992, my U.S. Pat. No. 5,436,556 issued Jul. 25, 1995 and my U.S. Pat. No. 5,432,755 issued Jul. 11, 1995.
Internal leaks in systems, however, are more difficult to detect because the sound that is generated at the leak point, due to turbulent flow or friction does not couple well with air to transfer the energy of the internal sound to the signal detector's airborne sensor. Accordingly, location of the leak becomes very difficult. In some circumstances, the sound generated by the leak is so weak that location is impossible. Another drawback of using a signal detection instrument's airborne sensor to locate internal leaks is that background noise or other competing sounds from the ambient environment flood the sensor effectively causing it to become insensitive to these lower sound levels generated from within the system.
Accordingly, where there is sound from an internal leak within an enclosure, such as a valve leak, a steam trap, or bearing friction, it is beneficial to employ a touch probe to detect the sound. In the past, ultrasonic leak detectors capable of detecting internal leaks have employed a touch probe mechanically coupled to an acoustic emission sensor so that the touch probe essentially acts as a conduit between sound traveling within a solid and the acoustic emission sensor. In use, a user would necessarily contact the solid with the touch probe in order to detect the sound and then convert this sound into an electrical signal with the acoustic emission sensor for further processing. For enclosures which are not under pressure, it is also known to utilize an ultrasonic signal generator in conjunction with an ultrasonic signal detector when investigating the existence of internal leaks. The generator is typically placed inside the enclosure and generates an ultrasonic tone which will follow the empty passage a gas or liquid would travel, thereby producing a leak. The point of exit of this ultrasonic tone can then be determined with the ultrasonic signal detector.
The majority of ultrasonic detectors prevalent in the market are capable of detecting either external leaks through the use of an airborne sensor or internal leaks through the use of an acoustic emission sensor, but not both. However, one device which is capable of detecting both internal and external leaks is described in my pending U.S. patent application Ser. No. 08/986,635 filed Dec. 8, 1997, and entitled "Signal Detector and Method for Detecting Signals Having Selected Frequency Characteristics". Here, the detector's housing accommodates both an airborne sensor for detecting external leaks and an acoustic emission sensor used in conjunction with a touch probe to detect internal leaks. While this integrated instrument has proven quite versatile, it does result in the use of additional components because each sensor requires its own associated pre-amplification circuitry in order to necessarily condition the incoming signals appropriately for the instrument. It, therefore, becomes necessary to recalibrate the instrument if the sensors are switched because it must appear to the instrument's processing circuitry that the same sensor is being employed. As a result, while such a detector is more versatile for monitoring leaks in a variety of environments, it is relatively expensive to produce and can be inconvenient as a result of the need to recalibrate the instrument due to the differing characteristics of the sensors.
Accordingly, there remains a need to provide a new and improved signal detection instrument for monitoring both internal and external leaks in systems, where the instrument is less expensive and does not require recalibration for each type of sensor. It would also be desirable to provide converter for use with the airborne sensor associated with existing ultrasonic signal detectors to provide this versatility by converting sounds traveling within a solid into airborne sound. The present invention is directed to meeting these needs.